A 0.050 M solution of the weak acid HA has [H3O+] = 3.77 x10-4M. What is the Ka for the acid?

A) 7.5 ? 10-3 M

B) 2.9 ? 10-6 M

C) 7.0 ? 10-7 M

D) 7.0 ? 10-8 M

E) 2.6 ? 10-11 M